The present invention concerns a composite electrode for electrochemical cell, an electrochemical cell comprising the composite electrode, a process for preparing the composite electrode, as well as a process for the preparation of a half-cell comprising a porous mineral compositexe2x80x94porous mineral separator sub-network.
More particularly, the invention concerns a composite electrode in which two solid interpenetrated electrolytes are used. The first electrolyte, of organic nature, preferably consists of a dry or gelled polymer which is made conductive by dissolving a salt or a mixture of salts preferably containing a lithium ion, and which acts as a deformable binder of the composite. The second electrolyte, of mineral nature, preferably vitreous, is a specific ion conductor, such as lithium ions, and in which the components of the first polymer electrolyte are insoluble. The solid mineral electrolyte is prepared in the form of an aqueous solution or in a mixture of water and light alcohols, which is contacted with the materials of the electrode in dispersed form so as to moisten at least in part the solid phases of the composite, i.e. the particles of the active material of the electrode, the additive of electronic conduction as well as the current collector of the electrode on which the dispersion is applied so as to form first a porous mineral sub-assembly. The organic electrolyte preferably comprising a polymer is thereafter introduced by permeation (impregnation) in the porous mineral sub-assembly so as to constitute the composite electrode of the invention.
The use of inert binders for preparing composite electrodes with liquid electrolytes has been known to one skilled in the art for a number of years. With the development of intrinsic or gelled conductive polymer electrolytes, the electrolyte has also become the binder of the composite electrode, which has made it possible to more easily adapt the variations of volume of the materials of the electrode. (Armand Europe and Bellcore). The use of appreciable amounts of mineral binders of polysilicates type in composites to mechanically bind the materials of the electrode to their current collector in a system of lithium-ion type with liquid electrolyte has also been described, and the possibility to carry out about twenty cycles has been demonstrated (Fauteux U.S. Pat. No. 5,856,045). A process of manufacturing a composite electrode including a step of preparing a porous electrode on its collector under air and a step of impregnating components of a polymer electrolyte so as to compensate for the porosity has been described in WO 97/44843, without however mentioning the preferred moistening of the phases in presence.
The treatment of active electrode materials with mineral phases to reduce the chemical aggressiveness of the active powders towards the organic electrolytes was also described for cathodes of lithium-ion systems (Tarascon).
Accessorily, the protection of current collectors such as aluminum with carbon base coatings was the object of numerous Patents which proposed the use of organic type inert binders (Fauteux U.W. U.S. Pat. No. 5,580,686).
None of these solutions meet all the criteria enabling to ensure the preservation of performances in power or cycling capacity, such as under conditions of elevated temperature, for example with polymer electrolyte lithium generators operating at temperatures between 40 and 80xc2x0 C., (JPS MG) or still with lithium-ion type generators operating at room temperature but under high voltages close to 4 volts (ref. lithiumion). In particular, none of the suggested solutions propose a global solution to the problem of preserving the quality of the ionic and electronic exchanges between all the phases constituting the composite electrode, such as between the dispersed solid particles. In the cases of extreme temperature and voltage, the collectors and the interfaces between the various solid phases, or between the particles of the same phase, are strongly solicited and have a tendency to prematurely lose their properties by formation of passivation films which are prejudicial to the quality of the electrical contacts inside the generator. Under these conditions, the components of the electrolytes (solvent and salt) play an important part in the formation of passivation films at the interfaces.
An advantageous way to maintain the quality of the electrical contacts in a generator would consist in using vitreous or crystalline solid mineral electrolytes which are in addition specific conductors of lithium ions. These solid electrolytes are less kinetically or thermodynamically reactive that organic liquids or polymers and do not require the use of a mobile lithium salt which can react with the active material of the electrode or with the current collector.
On the other hand, in vitreous state and when prepared in solution, these electrolytes have wetting properties which enable them to maintain at least part of the exchange surfaces during phenomena of passivation produced by a direct contact with the components of an organic electrolyte. These electrolytes however have disadvantages in that they are poor conductors and furthermore, they are rigid and brittle, i.e. they do not easily accommodate to variations of volume taking place at the electrodes during discharge/charge cycles. It is therefore difficult to use them as the only electrolyte of a composite and it is still more difficult to use them as binder of a composite which undergoes important variations of volume of the electrodes during cycling.
It is an object of the present invention to overcome these disadvantages by associating in a same composite electrode of a generator, the use of a solid mineral electrolyte with a solid organic electrolyte, preferably a polymer electrolyte.
It is another object of the invention to use first a solid mineral electrolyte, preferably vitreous or partially vitreous, in aqueous solution, to wet and adhere to at least part of the surface of the phases in presence: current collector, active material of the electrode and to at least part of the additive of electronic conduction.
It is also an object of the invention to provide a preparation step which defines impervious contact zones between the vitreous electrolyte and each of the solid phases.
It is also an object of the invention to disperse the active material of the electrode and the additive of electronic conduction in a aqueous phase comprising the solid vitreous electrolyte in solution and to apply same on the current collector so as to constitute a porous mineral subassembly.
It is also an object of the invention to provide a process which consists in impregnating a solid organic polymer electrolyte and a lithium salt both present in the form of molten polymer, polymer solution or still in the form of polymer which will be cross-linked after formation, so as to constitute a composite electrode with two solid electrolytes, one in the form of mineral glass, the other in the form of organic elastomeric polymer.
Another object of the invention resides in the use of the vitreous solid electrolyte to wet at least part of the surface of the solid phases in presence so as to cause these contact surfaces (entirely mineral) to be inaccessible to the components of the organic solid electrolyte (polymer and lithium salt) and thus to preserve this portion of the surface from the formation of passivation films which are harmful to the quality of electrical contacts.
It is still another object of the invention to optimize the electrochemical performances of at least one composite electrode of a rechargeable lithium generator when this generator operates under difficult conditions of temperature or high voltages and for long periods of time.
It is still another object of the invention to combine into one single composite electrode the advantageous properties of a solid mineral electrolyte with the complementary properties associated with solid polymer electrolytes.
Another object of the invention is to cause the vitreous solid mineral electrolyte to wet at least part of the solid phases so as to protect the zones thus coated, from corrosion and passivation phenomena resulting from an extended contact with the components of the organic electrolyte and thus to protect the quality of the electrical contacts between the different phases, such as with respect to ionic and electronic exchanges required for the performances of the generator.
Another object of the invention is to cause the organic polymer electrolyte to provide a higher level of conductivity as well as adjustable mechanical properties such as adhesiveness and a capacity of deformation which is essential as a binder which is capable of adjustment to volumic variations of the materials of the electrode during cycling.
Finally, it is another object of the invention to produce a composite electrode which is particularly useful for lithium metal dry polymer type generators operating under hot conditions between 40 and 125xc2x0 C. or for lithium ion type generators in which the voltages at the end of the recharge period are often higher than 4 Volts.
The invention concerns a composite electrode of an electrochemical cell, the latter including a second electrode and at least one electrolyte acting as separator, and disposed between the composite electrode and the second electrode. The composite electrode comprises a current collector, dispersed particles of electrochemically active material, possibly a first additive of electronic conduction and at least two solid electrolytes, namely a first electrolyte of mineral nature which is at least partly vitreous, and a second electrolyte of organic nature of polymer electrolyte type. The composite electrode is characterized in that the electrolyte of mineral nature at least partly wets the surfaces of the active material dispersed in the composite electrode, of the conduction additive found therein as well as of the current collector so as to constitute a porous mineral sub-assembly. This surface, which is partly wetted, is thus protected by means of an impervious mineral phase, and the second electrolyte of organic nature of polymer electrolyte type is in close contact with the porous mineral sub-assembly so as to ensure ionic exchanges within the composite electrode and with the other components of the cell. The second electrolyte of organic nature of polymer electrolyte type additionally acts as deformable binder of the assembly consisting of the composite electrode, its collector and the separator of the cell, and possibly comprises a second additive of electronic conduction in dispersed form enabling to optimize electronic exchanges within the composite electrode and it current collector, the composite electrode including at least two solid electrolytes, thus ensuring zones where at least part of the exchange surfaces between solid phases and the electrolyte of mineral nature at least partly vitreous is non accessible to the components of the electrolyte of organic nature of polymer electrolyte type and to contaminants which may originate from other components of the electrochemical cell and are less susceptible to form films of passivation or degradation, thereby ensuring electrical contacts of good quality within the composite electrode, in spite of difficult conditions of temperature, voltage or cycling.
The first additive of electronic conduction is preferably selected from carbon blacks, graphites, metallic powders, doped conjugated polymers, nitrides or carbides of transition metals. With respect to the second additive of electronic conduction, it may comprise carbon blacks and graphites. On the other hand, the electrolyte of mineral nature at least partly vitreous may be selected from the group consisting of phosphates or polyphosphates and borates or polyborates of an alkali metal, such as lithium or potassium, and mixtures thereof, which are conductors of alkaline ions.
The electrolyte of mineral nature may include glass forming elements for example those selected from silicas, siloxanes, completely or partially hydrolyzed aluminates and titanates.
The electrolyte of mineral nature consists for example of lithium phosphate of approximate stoichiometry LiPO3, whose pH is between 4 and 9 et having a minimum conductivity of 10xe2x88x9210 S.cmxe2x88x921 at the temperature of operation of the electrochemical cell. This lithium phosphate is prepared by fast neutralization of metaphosphoric acid in water solution with a lithium salt to give a pH between 4 and 9.
The electrochemical material which is present in the composite electrode is for example selected from oxides, chalcogenides and oxyanion derivatives of transition metals, and of lithium or mixtures thereof. For example, it is based on vanadium oxide, phosphates or phosphosulfates of olivine structure of transition metals comprising iron or manganese of Nasicons, such as iron phosphates of the type LiFePO4 in discharged state.
The additive of electronic conduction may comprise carbons or graphites, it may be of metallic type, in dispersed form, for example silver, copper, or may be of semi-metallic type, in dispersed form, for example it may be selected from carbides, nitrides, borides of metals or doped conjugated polymers. The current collector may comprise aluminum or copper.
According to a preferred embodiment, the active material of the composite electrode is at a voltage higher than 1.6 volts with respect to that of metallic lithium and operates as an anode.
On the other hand, it is preferred but not essential that the volumic fraction of the electrolyte of mineral nature which is at least partly vitreous with respect to the dispersed phases of active material and additive of electronic conduction be comprised between 2 and 25% so as to permit an at least partial wetting of the dispersed solid phases, the formation of an open porosity of the mineral sub-assembly, and the adhesion of the electrolyte of mineral nature at least partly vitreous to the collector before introducing the electrolyte of organic nature of polymer electrolyte type. Moreover, the open porosity of the sub-assembly is preferably about 30 to 70%.
The electrolyte of organic nature of polymer electrolyte type preferably includes a soluble salt to make it conductive, for example a lithium salt.
According to another preferred embodiment, the electrolyte of organic nature of polymer electrolyte type mainly consists of a polyether including ethylene oxide units; or it may comprise a gel consisting of a polyether matrix or a polyelectrolyte, a liquid aprotic solvent and a soluble salt. It may also comprise a gel consisting of a non solvating polymer matrix, a liquid aprotic solvent and a soluble salt, for example the gel may consist of a non solvating polymer matrix, a solvent comprising an aprotic compound and a soluble lithium salt.
According to another embodiment, the electrolyte of organic nature of polymer electrolyte type is present within the electrolyte of mineral nature so as to constitute a deformable binder for the composite electrode.
The separator of the electrochemical cell according to the invention normally consists of a solvating or non solvating solid polymer electrolyte, or a polymer, an aprotic solvent and a dissociated lithium salt. The separator may also consist of a solid powder which is bound by an electrolyte of mineral nature which is at least partially vitreous according to the invention, which is impregnated with a liquid or polymeric electrolyte.
The invention also concerns a process for preparing a composite electrode of an electrochemical cell, including two locally distributed solid electrolytes, the first one of mineral nature at least partly vitreous, the second one of organic nature of polymer electrolyte type, as well as its collector. The process is characterized by preparing an aqueous solution of the first electrolyte or its precursor in which particles of electrode material and possibly an additive of conduction are dispersed, applying the dispersion obtained on a current collector and drying the dispersion so as to wet and at least partly coat the particles, the additive of electronic conduction and the current collector, and to constitute a porous mineral sub-assembly which is sufficiently adherent to the current collector to enable it to be handled in the form of a film, and thereafter possibly applying a dispersion of an additive of electronic conduction in an electrolyte of organic nature of polymer electrolyte type containing a lithium salt on the film of the porous mineral sub-assembly so as to completely or partly introduce within the porous mineral sub-assembly the electrolyte of organic nature of polymer type so that it acts as binder and mixed conductor inside the porous sub-assembly, thereby obtaining the composite electrode.
The particles of material include for example a first additive of electronic conduction also dispersed in the aqueous solution of the first electrolyte so as to give after evaporation, a mixed coating which is conductive towards lithium ions and electrons. The electrolyte of organic nature of polymer electrolyte type may be introduced into the porous mineral sub-assembly in molten form, as a solution in a solvent, or in the form of prepolymer or monomer of low molecular weight, which is cross-linked or polymerized after formation.
Preferably, the mineral electrolyte is at least partly vitreous and impervious to the components of the organic electrolyte of polymer electrolyte type, and is selected from phosphates and polyphosphates, borates and polyborates of an alkali metal such as lithium or potassium and mixtures thereof.
The organic electrolyte is preferably a solvating or non solvating polymer, or a non solvating polymer in gelled or non gelled form. It may also comprise a polar aprotic solvent, a dissolved lithium salt and a dispersed additive of electronic conduction, or it may mainly consist of a polyether including ethylene oxide units.
The invention also concerns a process for preparing a half-cell characterized by preparing an aqueous solution of a mineral electrolyte or its precursor, in which particles of an electrode material and an additive of electronic conduction are dispersed, applying the dispersion obtained on a current collector and drying the dispersion so as to wet and at least partly coat the particles and the current collector, and to constitute a porous mineral composite sub-assembly which is sufficiently adherent to the current collector to permit its handling in the form of film, preparing a dispersion of a mineral powder in another aqueous solution of the mineral electrolyte or its precursor and applying it on the porous mineral composite sub-assembly so as to constitute a porous half-cell including the collector, the mineral composite sub-assembly and a porous mineral separator, and applying a polymer electrolyte containing a lithium salt on the porous half-cell so as to completely or partly introduce the polymer electrolyte in all the porous components, the latter then serving as binder and conductor in the porous mineral compositexe2x80x94porous mineral separator composite sub-assembly, thus forming a half-cell with double solid electrolytes.
The mineral powder is preferably selected from insulating oxides and solids which are conductors of lithium ions, or Nasicons.
A half-cell may also be prepared by applying a polymer electrolyte separator on a composite electrode according to the invention, the polymer electrolyte being then loaded with mineral powders which are conductive or non conductive with respect to lithium ions.
The polymer electrolyte separator may be applied by hot pressing, in the form of a molten liquid phase, in dissolved form or as a prepolymer.
The invention also concerns a process of manufacturing an electrochemical cell characterized by assembling a composite electrode according to the invention with a separator as defined above, and adding a second electrode. In this case, the composite electrode is a cathode based on vanadium oxide or transition metal phosphate of olivine structure, in which the collector is aluminum, and the second electrode may be a lithium anode.